The motor control center (MCC) includes a basic unit each constituted of a circuit breaker, a switchgear, a protection control module, etc. to control, stop, and protect the motor and an inlet unit constituted of a main line for supplying power to a branch line of each basic unit and a circuit breaker (See FIG. 1).
The protection control module, which is one of the components constituting the basic unit, may be configured separately as a protection device and a control device and is applied to various products ranging from a device of simple function to a device of high function with a communication function.
A digital protection control module having a communication function has been widely used, as it is necessary to be controlled in front of the MCC panel and controlled remotely from the main control room in the case of a factory or a building (refer to FIG. 2).
On the other hand, for the purpose of connection with the local operating panel (LOP) responsible for the operation of the motor at a local place where the motor is installed, analog signal lines are usually installed per a basic unit (see FIG. 3).
In order to check the current flowing through the motor in the LOP, the CT installed in the basic unit should be connected to the ammeter of the local operating panel. In order to display the operation status (RUN, STOP, etc.) of the motor on the local operating panel, digital output (DO) terminals of the protection control module in the basic unit are connected to the display lamps of the local operating panel. In order for operation command such as RUN, STOP, and the like obtained by pressing push buttons in the local operating panel to control the magnet contactor (M/C) of the protection control module through digital input (DI) terminals of the protection control module, the push buttons in the local operating panel have to be connected to the DI terminals of the protection control module.
However, since such analog signal lines have to be installed between each basic unit and the corresponding local operating panel, it is obvious that the installation cost is high and maintenance is difficult. In FIG. 3, although it is shown that only one basic unit is connected to the local operating panel for the sake of convenience, in reality, this connection is required between all the basic units and the local operating panel, whereby a large number of cables are required.
In order to solve the problem of connecting LOPs, which are typically several tens to several hundreds of meters away from the motor control board, using the analog signal lines, a method may be considered in which a device of the LOP is used with a digital device having a serial communication function and is connected to the protection control module of the basic unit via communication.
In addition, it is noted to think about a method in which upon connecting all the LOPs at once via serial communication, a serial communication device installed in the motor control center performs communications with the LOPs and distributes and assembles the communications to and from the protection control module of each basic unit.
However, in the above methods, a signal line and a communication line are required between the serial communication device of the motor control center and the protection control module of each basic unit. According to the present invention, a new type module called a ‘integrated module’ is installed in the motor control center, a serial communication network is installed between the integrated module and each protection control module, and in particular, this serial communication network is commonly used between the LOPs and each protection control module and between the LOPs and a computer or a PLC in a main control room, thereby increasing efficiency in the overall system configuration surrounding the motor control center.
However, as the digital equipment that performs serial communication is included in the LOP, unlike analog type LOP in the related art, the digital LOP needs power supply. Since the power supply is improperly out of the power supply system for operation of the motor control center when power supply in the vicinity of the local place is used, it is desirable to draw power supply from the motor control center.
Since the control power of the LOP cannot be obtained from the local place where the LOP is installed, it is needed to be connected to power supply in the motor control center. As shown in FIG. 4(a), the LOP may receive power using a single power line in parallel. However, when a power line is disconnected at one point, there is a problem that power cannot be supplied to the LOP that is at a stage subsequent to the point where the disconnection occurs.
In order to solve this problem, an individual power line may be connected to each LOP as shown in FIG. 4(b). In this case, however, a large number of power lines, which are proportional to the number of LOPs, has to be connected. There is a problem of losing the advantage in a method of connecting all LOPs via serial communication. Although the number of signal lines is reduced as compared with the analog signal line, the power line must be connected to each LOP, thereby having no almost advantage of efficiency.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments.